An organic light emitting diode (OLED) is a current-mode light emitting device. Due to advantages such as spontaneous light emitting, fast response, wide-viewing angle and ability to be fabricated on a flexible substrate, the OLED is more frequently used in a field of high performance display. OLEDs can be classified into two kinds of passive matrix driving OLEDs (PMOLEDs) and active matrix driving OLEDs (AMOLEDs) based on driving modes thereof. With respect to traditional PMOLEDs, with increasing of size of a display apparatus, a driving time of a single pixel is usually required to be reduced, thus a transient current flowing through a PMOLED is required to be increased, and thereby power consumption will be increased significantly. In contrast, with respect to AMOLEDs, as a current is inputted into each OLED by progressive scanning of a thin film transistor (TFT) switching circuit, existing problems can be well solved.
In an existing AMOLED pixel driving circuit, voltage of a gate of a driving transistor during a light emitting stage can be held in three ways. The first way is to improve current leakage property of transistors, that is, to reduce current leakage of transistors. The second way is to use a design of a pair of transistors so as to be connected with the gate of the driving transistor. The third way is to increase the capacitance of a storage capacitor (C). A present AMOLED pixel driving circuit usually employs the three ways as above.
Specifically, FIG. 1 shows a configuration of an AMOLED pixel driving circuit commonly used in current market. In this pixel driving circuit, in order to suppress outflow of charges from a gate of a driving transistor M8 during a light emitting stage, capacitance of a storage capacitor C is increased, and a pair of transistors are used to connected with the gate of the driving transistor M8, that is, transistors M1 and M2 are formed as a pair of transistors, and transistors M3 and M4 are formed as a pair of transistors. In this way, during the light emitting stage of the AMOLED, potential of the gate of the driving transistor M8 is unchanged, and light emitting is stabilized. The operational principle of the pixel driving circuit is briefly described as follows. The pixel driving circuit operates in three stages. A first stage is a reset stage, in which a signal of Vref is effective, and the transistors M1 and M2 are turned on, thus potential between the storage capacitor and the gate of the driving transistor M8 is reset by a signal of Vin through the transistors M1 and M2. A second stage is a data write stage, in which the signal of Vref becomes ineffective, an effective signal is inputted through a gate line Gate, and a data signal Data is inputted to a source of the transistor M8 through a transistor M5, then the transistors M3 and M4 are turned on so that the gate and a drain of the transistor M8 are connected with each other, that is, the transistor M8 functions as a diode, thus a signal Data+Vth is written to the gate of the transistor M8, that is, is written into the storage capacitor C to be held by the capacitor, wherein Vth is a threshold voltage of the transistor M8. A third stage is a light emitting stage, in which both of Vref and Gate become ineffective, and an effective signal is inputted from a emission signal input terminal Em, thus transistors M6, M7 and M8 are turned on, thereby an organic light emitting diode D emits light, then when potential of the gate of the transistor M8 is constant, the organic light emitting diode D emits light stably, at this time, potential of the gate of the transistor M8 is held by the storage capacitor C and the pair transistors M1, M2 and M3, M4.
In this configuration, the larger the capacitance of the storage capacitor C is, the better the potential of the gate of the driving transistor M8 is maintained. However, in a high resolution product, size of a pixel is smaller and smaller, resulting that the storage capacitor will not be large, which will directly affect potential holding effect.